The recent increase in importance of the photovoltaic power generation in response to the emerging energy resource problems and global environmental problems has led to active expansion of the photovoltaic power generation and development of energy supply technologies.
The photovoltaic power generation is aimed at achieving the level of costs for general-purpose power in terms of cost efficiency. Accordingly, solar cells producible at a reduced module production cost and having a high generating efficiency have been desired.
To increase the generation efficiency, the light-concentration efficiency of the solar battery cell can be increased. Conventional optical parts for light concentration are, for example, (1) lenses (spherical or aspheric lenses), (2) concave mirrors, and (3) plane films.
The plane films developed are, for example, optical parts such as Fresnel lenses as described in Patent Literature 1. A conventional plane film, however, cannot achieve highly efficient concentration of a ray of light that strikes the film at a shallow angle from the in-plane direction of the film. For this reason, if the plane films are used as light-concentrating films for a solar cell, the battery needs to be, for efficient power generation, a sun-tracking solar cell which adjusts the orientation of solar battery cells to the direction of sunlight irradiation. Since sun-tracking solar cells are systems controlled to have focusing elements always face the sun, the overall structure of the systems is complicated to cause cost- and equipment-related problems.
Another known plane film is an optical element using a hologram. For example, a hologram produced by fixing a hologram diffraction pattern onto a photosensitive material through laser interference is used. Such a hologram diffraction pattern is formed by a method of recording the interference fringes of the reference light (e.g. laser light) and object light onto a photosensitive material (e.g. dichromated gelatin), and optically fixing the complex amplitude information of the object light on the recording surface.
As well as such a hologram formed by fixing a hologram diffraction pattern onto a photosensitive material through laser interference, computer generated holograms (CGHs) formed by calculating diffraction patterns using a computer have been developed.
Patent Literature 2 discloses an optical branch having at least one first optical system; a hologram element with a diffraction grating pattern formed to branch the radiating light emitted from the first optical system into a plurality of beams and to converge the beams to a plurality of spots; and a plurality of second optical systems with light receivers disposed at positions corresponding to the spots. Here, the optical phase of the diffraction plane of the hologram element can be designed by computer-generated holography using a computer. These conventional hologram elements, however, cannot concentrate rays of light striking at various angles.
A known method of reducing the reflection loss to efficiently catch the rays of sunlight at various angles is forming a moth-eye (bug-eye) structure on the surface of a film. This method forms fine, transparent objects such as cones, triangular pyramids, and quadrangular pyramids on a surface of a film to reduce the reflection loss and efficiently concentrate external light rays. With this method, however, the capture rate (transmittance) of the sunlight greatly falls at an angle to a horizontal plane of smaller than 100, leading to inefficient concentration of the sunlight. Also, the surface of the film is easily contaminated to damage the durability. The method does not have favorable productivity either.